In the closed state the front surfaces of the two halves of the mould are in contact with each other along the parting plane, or along the parting surface if the shape of the object to be produced precludes the use of a flat plane. Each half of the mould consists of a retaining plate which is attached to the injection moulding machine, as well as a mould insert which is let into the very robustly designed retaining plate; and the front surface of the insert, which must align with the parting surface, contains in negative form part of the contour of the object to be produced. The quality and dimensional accuracy of the objects to be produced are determined to a decisive extent by the parallel alignment of the two opposed mould inserts. This alignment can be achieved only by carrying out exact adjustment relative to the respective associated retaining plate which in turn is connected in a defined manner with the injection moulding machine.
In the past, this necessarily exact locating of the mould insert in the retaining plate was achieved by ensuring that the complete outer contour of the mould insert corresponded exactly with the associated bearing surfaces in the cavity on the retaining plate. Usually, the mould insert was manufactured as a turned part whose outer diameter corresponded exactly to the inner diameter of the likewise cylindrically configured cavity, and its axial length corresponded exactly to the depth of the cavity provided in the retaining plate.
Tolerances of 1/100 mm and even tighter have to be met by such moulds and, in addition, allowance must be made for the thermal expansion of the individual parts when the mould is used as an injection moulding tool. As a result, it was common for a large number of the moulds to be rejected as unacceptable right at the manufacturing stage.
When a mould insert is manufactured, first its rear bearing surface and its peripheral surface are manufactured to the correct size. Once they have been finish-machined, these surfaces are used as reference surfaces when the insert is clamped in another machine tool on which the negative shapes on the front side of the mould insert are machined out. However, if there is a slight deviation in dimensions during the machining, or if the dimensionally accurate contour is merely offset from the desired location, this can no longer be compensated for by reworking the rear surface or the peripheral surfaces of the mould insert, because the latter would then no longer fit tightly and without play in the associated retaining plate.
Furthermore, replacing such mould inserts in the retaining plate is a very time-consuming task, because in part this can only be done after the retaining plate has been removed from the injection moulding machine; and, because of the very small amount of play which is present following the build-up of dirt, etc., particularly after long periods of use, coupled with the need to handle the mould carefully, it sometimes takes a very long time to remove the mould insert and to fit it into the retaining plate. On the other hand, however, because of the high capital investment involved, an injection moulding machine can only be economically used if the downtimes, which are generated by among other things the replacement of the tools, can be kept to a minimum.
In addition, if any heat distortion occurs, it is no longer possible to correct the position of the mould insert in the retaining plate.
Another disadvantage of these moulds was that the retaining plates always had to be designed to accept the maximum possible axial dimension of the parts to be manufactured. Even when short parts were produced, it was still necessary to use a mould insert with the maximum elongation matching the cavity of the retaining plate. Not only did this result in high material costs for the mould insert, but also the absolute thermal distortion was increased due to the long axial dimension.